The discussion throughout this specification comes about due to the realisation of the inventors and/or the identification of certain prior art problems by the inventors.
In recent years limits have been imposed on the strength of signal transmissions in RFID systems for various reasons. However, such a reduction in signal strength has also brought with it increased problems with the reliability of signal transmission. Because the signal strength is reduced due to regulations imposed in various countries, the signals transmitted are increasingly susceptible to noise and other external factors and become increasing difficult to receive and decode, any one of which can interfere with reliable signal transmission.
FIG. 1 illustrates a ‘spectral mask’ being the spectrum field strength limits as defined by CEPT/cal recommendation 70-03[3]; Draft ETSI EN300 330-1 V1.4.1 (2004-11). The spectral mask shown results in 99%, or more, of the signal power being in the central carrier and 1%, or less, of the signal power being in the modulated sidebands. This type of signal is quite unique as normally the power in the modulated sideband (which conveys the data) is as large as possible and the power in the central carrier is a small as possible and ideally is zero as it conveys no data.
FIG. 2 illustrates a representation of a signal according to ISO 15693 standard which meets the spectral limits of FIG. 1. However, this transmission is at 26 Kbit/s, 13K Baud and is thus considered to have a relatively slow bit rate and relatively slow baud rate. Accordingly, this transmission standard is not considered suitable for a number of applications, such as RFID application in which high speed RF signal transmissions are required.
FIG. 3 illustrates a method of transmission as used by the current applicant in conjunction with their Phase Jitter Modulation (PJM) transmission technique as disclosed in WO1999034526, the disclosure of which is incorporated herein by reference. This transmission mode ulitises 1 degree of phase modulation, at 424 Kbit/sec, and has been found to be very effective in RFID applications. However, it would be desirable to take advantage of the spectral mask limits and have more of the signal as illustrated in FIG. 3 within the region marked A, i.e. between the levels of −33 dB and −45.5 dB. The result allow a boost in modulation signal strength of substantially 12.5 dB (approximately 1 degree to 3 degrees), but still be substantially within the spectral mask limits as illustrated in FIG. 1.
FIG. 4 illustrates an alternative method of transmission to the PJM transmission of FIG. 3. This alternative method utilizes 3 degrees phase modulation which enhances transmission reliability because the received signal is substantially three times stronger and hence the signal to noise ratio is much better than a signal having one degree phase modulation. It can be seen that this alternative transmission has a relatively increased portion of the signal within the region A of the spectral mask (as compared to FIG. 3), and in this regard, this alternative transmission has a boosted signal strength as compared to the transmission illustrated in FIG. 3. However, the alternative transmission operates at only half the data rate of the transmission of FIG. 3, i.e. at a data rate of 212 kbit/sec. It is considered desirable to have an improved bit rate.
FIG. 5 illustrates a relatively conventional MFM method of transmission, at 424 kbit/sec and as applied to the spectral mask of FIG. 1. By way of explanation, MFM or modified frequency modulation transmission improves on FM (Frequency modulation) by reducing the number of phase reversals inserted just for the clock. Instead of inserting a clock reversal at the start of every bit, one is inserted only between consecutive zeros. When a 1 is involved there is already a reversal (in the middle of the bit) so additional clocking reversals are not needed. When a zero is preceded by a 1, it is similarly known that there was recently a reversal and another is not needed. Only long strings of zeros have to be “broken up” by adding clocking reversals. FIG. 6 provides an illustration of MFM method of transmission, where FM and MFM encoding write waveform for the byte “10001111”. It can be seen that MFM encodes the same data with substantially half as many edges per bit of data. In FIG. 5 it can be seen that a portion of the signal is desirably within the region marked A (compared to FIG. 3 which has little signal in region A, and FIG. 4 which has a comparatively slow data rate). However, the transmission of MFM is considered susceptible to noise and other external factors and impact on the reliability of the signal transmission. It would be desirable to have a relatively more reliable method of signal transmission.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.
An object of the present invention is to provide a transmission method and/or associated device which enables signal transmission at relatively high data rates and within a prescribed spectral mask, for example the spectral mask of FIG. 1.
A further object of the present invention is to alleviate at least one disadvantage associated with the prior art.